Process for selectively etching a layer of silicon dioxide on an underlying stop layer of silicon nitride

ABSTRACT

More specifically, a process is provided for etching a multilayer structure to form a predetermined etched pattern therein. The subject process comprises providing the multilayer structure having a plurality of structural layers. The structural layers of the multilayer structure comprise a silicon dioxide outer layer on an underlying silicon nitride stop layer. Then, a chemical etchant protective layer is formed on a major surface of the multilayer structure having a predetermined pattern of openings, thereby exposing areas of the silicon dioxide outer layer corresponding to the predetermined pattern of openings. The exposed areas of the silicon dioxide outer layer are then etched down to the silicon nitride stop layer, at a high SiO 2  etch rate and at a high level of selectivity of the SiO 2  etch rate with respect to the Si 3  N 4  etch rate, with a fluorinated chemical etchant system. The fluorinated chemical etchant system includes an etchant material and an additive material. The additive material comprises a fluorocarbon material in which the number of hydrogen atoms is equal to or greater than the number of fluorine atoms. The etching step forms a substantially predetermined etch pattern in the silicon dioxide outer layer in which the contact sidewalls of said SiO 2  outer layer are substantially upright.

BACKGROUND OF THE INVENTION

This invention relates to a process for selectively etching a silicondioxide layer deposited on a silicon nitride layer, and moreparticularly to a process for effectively and efficiently etching suchsilicon dioxide layer at a high etch rate and high selectivity ofsilicon dioxide with respect to silicon nitride, particularly in amultilayer structure.

It is known in the prior art that the manufacture of multilayermultilayer structures typically involves patterned etching of areas ofthe semiconductor surface which are not covered by a pattern ofphotoresist protective material. These etching techniques use liquid orwet etching materials, or dry etching with halogens orhalogen-containing compounds, of certain layers of these devices. Forexample, one well known etching material is chlorine which can exist inthe etching process as either chlorine gas or HCl, etc. Chlorine etchesthe semiconductor isotropically, i.e., in both a lateral and verticaldirection. This results in an etched feature which has a line widthwhich is smaller than the exposed resist image.

Etching of the multilayer structures can also be conducted in a gasphase using known techniques such as plasma etching, ion beam etching,and reactive ion etching. The use of gas plasma technology providessubstantially anisotropic etching using gaseous ions, typicallygenerated by an RF discharge. In gas plasma etching the requisiteportion of the surface to be etched is removed by a chemical reactionbetween the gaseous ions and the subject surface. In the anisotropicprocess, etching takes place only or primarily in the vertical directionso that feature widths substantially match the photoresist patternwidths. Anisotropic etching is utilized when feature sizing afteretching must be maintained within specific limits in order not toviolate alignment tolerances or design rules. For example, in U.S. Pat.No. 4,734,157 an elemental silicon-containing layer, such as a layer ofpolysilicon or silicide, is etched anisotropically employing a gasplasma comprising a gaseous chlorofluorocarbon, capable of supplyingCF_(x) and chlorine ions, and ammonia. Profile control of a siliconlayer is controlled by the use of this etching mode.

Higher density multilayer structures such as 64 and 256 Megabit DRAMwill require an additional amount of alignment tolerance which can notbe addressed by photolithography means. In such applications, an etchstop technology could be used to supply the desired tolerance. In anetch stop system an etch stop layer is deposited on underlyingstructures. The outer layer is deposited over the underlying etch stoplayer through which the desired patterns will be defined. The etch stoplayer will then be used to terminate the etch process once the outerlayer has been completely removed in the desired pattern locations. Thusthe etch stop layer acts to protected structures underlying the etchstop layer from damage due to the outer layer dry chemical etch. Theprocess used to perform this etch must have three basic properties,namely, (1) a high outer layer etch rate which (2) producessubstantially upright sidewalls and (3) has a high selectivity of theouter layer being etched down to the etch stop layer. The preferred etchstop material is silicon nitride because it's properties are well knownand it is currently used for semiconductor fabrication. The preferredouter layer is silicon dioxide.

With respect to etching of a multilayer structure including a silicondioxide layer on an underlying silicon nitride layer, a problem whichoccurs and which must be overcome is profile control. Prior art methodsof obtaining high oxide to nitride selectivity rely on pure chemicaletching (such as hydrofluoric acid). Profile control using this methodproduced structures that do not have vertical sidewalls. Dry etchprocessing usually produces a more vertical profile because of the ionbombardment aspect of the process. However, the dry etch process canproduce a contact wall that slopes out from the bottom instead of being90 if the wrong mix of process parameters are used. These parameters caninclude, but are not limited to, CF₄, CHF₃, RF Power, and pressure.

The same ion bombardment aspect of the dry etch process used to producestraight sidewalls has a very negative effect on oxide to nitrideselectivity. High energy ions needed to etch both oxide and nitride doso by disassociating a chemical bond at the oxide and/or nitridesurface. However the disassociation energy needed for nitride is lessthan that required for oxide. Hence the addition of CH₂ F₂ to offset thedisassociation properties of nitride as compared to oxide. The CH₂ F₂produces a polymer deposition on the nitride surface that acts topassivate the nitride surface and thereby reduce the dry etch removalrate. However, the silicon dioxide etch rate is sustained at a muchhigher rate than that of silicon nitride.

Here is a discussion of various prior art processes for etching silicondioxide and/or silicon nitride. In U.S. Pat. No. 4,789,560 to Yen, forexample, a fusion stop method is provided for forming silicon oxideduring the fabrication of integrated circuit devices. A diffusion stoplayer of thermal silicon oxide is formed during the fabrication ofintegrated circuit device prior to the deposition of the poly layer tobe oxidized. The nitride isolates the substrate from diffused oxygenwithin the poly layer during oxidation, permitting a non-criticaloxidation time.

U.S. Pat. No. 4,877,641 to Dory discloses a plasma CVD for formingsilicon nitride or silicon dioxide films onto a substrate using areactant gas including di-tert butylsilane and at least one otherreactant gas.

U.S. Pat. No. 4,324,611 to Vogel et al. discloses a process and gasmixture for etching silicon dioxide and/or silicon nitride in a plasmaenvironment in a planar reactor using a carbon fluorine gas comprisingC₂ F₆, CF₄, C₃ F₈, C₄ F₁₀, C₄ F₈, and combinations thereof.

U.S. Pat. No. 4,912,061 to Nasr discloses a method of forming asalicided self-aligned metal oxide multilayer structure using adisposable silicon nitride spacer.

U.S. Pat. No. 4,568,410 to Thornquist relates to the selective gaseousplasma etching with nitrogen fluoride and an oxygen source gas ofsilicon nitride in the presence of silicon oxide.

U.S. Pat. No. 3,479,237 to Bergh et al. discloses etching silicon oxideon silicon nitride using a hydrofluoric acid solution.

U.S. Pat. No. 4,971,655 to Stefano et al. discloses a method forprotecting a refractory metal silicide during high-temperatureprocessing using a dual-layer cap of silicon nitride on silicon dioxide.

U.S. Pat. No. 5,013,398 to Long et al. discloses a plasma etch processto anisotropically etch a sandwich structure of silicon dioxide,polycrystalline silicon and silicon dioxide "in situ", that is, in asingle etch chamber.

U.S. Pat. No. 5,040,046 to Chhabra et al. discloses a process forforming silicon dioxide, or silicon nitride layers on selectedsubstrates employing C₄ H₁₂ Si and an O₂ source.

U.S. Pat. No. 5,013,692 to Ide et al. discloses a process for preparingfilm for a semiconductor memory device which comprises forming a siliconnitride film over a substrate by a chemical vapor deposition technique,oxidizing the surface of the silicon nitride film to form a siliconoxide layer over the film, and removing the silicon oxide layer byetching to form an improved silicon nitride film.

U.S. Pat. No. 4,244,752 to Henderson, Sr. et al. discloses a method offabricating an integrated circuit wherein a silicon oxide-siliconnitride layer is formed on the surface of a silicon wafer.

U.S. Pat. No. 4,374,698 to Sanders, et al. relates to the etching ofSiO₂ or Si₃ N₄ with CF₄, CF₂ C₁₂ or CF₃ Br, and O₂, while U.S. Pat. No.4,581,101 to Senoue et al. etches the same materials with a fluorinatedether.

U.S. Pat. No. 5,043,790 to Butler uses upper and lower nitride layers inthe formation of sealed self-aligned contacts. The upper non-conductivenitride layer is composed of silicon nitride which acts as an etch stoplayer for an isotropic silicon dioxide wet etch. The lower nitride layeris a titanium nitride layer on a titanium silicide layer, both of whichare conductive materials. The titanium nitride layer acts as an etchstop during an anisotropic dry etch of the silicon dioxide layer.

Current etch process technology for etching an SiO₂ outer layer on anunderlying Si₃ N₄ layer using a dry etcher, such as an RIE or MRIEetcher, cannot produce SiO₂ -to-Si₃ N₄ selectivities above 3:1 withadequate profile and SiO₂ etch rate characteristics. Therefore, a needexists for a process for etching a SiO₂ layer on an underlying Si₃ N₄layer, at a high SiO₂ etch rate, and at a high selectivity of SiO₂ withrespect to the underlying Si₃ N₄, to form an etched multilayer structureat a controlled predetermined profile in which the sidewalls aresubstantially upright.

SUMMARY OF THE INVENTION

The process of the present invention meets the above-described existingneeds by forming the above-described etched multilayer structure inwhich the sidewalls of the SiO₂ layer are substantially upright at ahigh SiO₂ etch rate and at a high selectivity of SiO₂ with respect tothe underlying Si₃ N₄. This is accomplished by employing a process foretching the SiO₂ layer down to the Si₃ N₄ stop layer as hereinafterdescribed.

In two published articles "Crown-Shaped Capacitor Cell for 1.5 VOperation 64 Mb DRAMS" by T. Kaga, et al in IEEE Transactions OnElectron Devices, Vol. 38, No. 2, February 1991, and "VSLI DeviceFabricator Using Unique, Highly Selective Si₃ N₄ Dry Etching" by T.Kure, et al Proceeding of the International Electron Devices Meeting(IEDM), 1983, pp. 757-759., a highly selective anisotropic dry etchingtechnique is described for etching a Si₃ N₄ layer down to an underlyingSiO₂ stop layer using a CH₂ F₂ plasma. However, applicants haveunexpectedly discovered that when CH₂ F₂ is employed as an additive in afluorinated chemical etchant system, substantially high oxide to nitrideselectivities can be achieved, with high etch rate, and substantiallyupright sidewall profiles.

More specifically, a process is provided for etching a multilayerstructure to form a predetermined etched pattern therein. The subjectprocess comprises providing the multilayer structure having a pluralityof structural layers. The structural layers of the multilayer structurecomprise a silicon dioxide outer layer on an underlying silicon nitridestop layer. Then, a chemical etchant protective layer is formed on amajor surface of the multilayer structure having a predetermined patternof openings, thereby exposing areas of the silicon dioxide outer layercorresponding to the predetermined pattern of openings. The exposedareas of the silicon dioxide outer layer are then etched down to thesilicon nitride stop layer, at a high SiO₂ etch rate, and at a highlevel of selectivity of the SiO₂ etch rate with respect to the Si₃ N₄etch rate, with a fluorinated chemical etchant system. The etching stepforms a substantially predetermined etch pattern in the silicon dioxidelayer in which the contact sidewalls of said SiO₂ are substantiallyupright.

The fluorinated chemical etchant system includes an etchant material andan additive material. The additive material comprises a fluorocarbonmaterial in which the number of hydrogen atoms is equal to or greaterthan the number of fluorine atoms. Fluorocarbon materials comprisecarbon, hydrogen and fluorine atoms in differing relative ratios. Forexample, the preferred fluorocarbon material employed as the additivematerial is CH₂ F₂. In case of CH₂ F₂, the number of hydrogen atoms (2)is equal to the number of fluorine atoms (2). Another fluorocarbonmaterial which can be used as the additive material in the presentinvention is CH₂ F₂. As to CH₃ F, the number of hydrogen atoms (3) isgreater than the number of fluorine atoms (1).

In the process of this invention the fluorinated chemical etchant systempreferably comprises from about 70-90%, and more preferably from about75-85%, of the etchant material, and from about 10-30%, and morepreferably from about 15-25% of the additive material, based on thetotal flow of the fluorinated chemical etchant system. The amount of theadditive material, CH₂ F₂, based on the total flow of fluorinatedchemical etchant system, is preferably at least about 3%, morepreferably at least about 12%, and most preferably at least about 20%.Preferably, the etchant material of the fluorinated chemical etchantsystem of this invention comprises at least one of CHF₃, CF₄ and Ar. Inthe preferred CHF₃ -Ar-CF₄ system, the amount of CHF₃ in the gas flowmixture is preferably about 3%, more preferably about 6%, and mostpreferably at least about 10% of the total gas flow. With respect toargon, the flow rate should be at least about 33%, more preferably atleast about 50%, and most preferably at least about 60% of the total gasflow. Finally, as to the flow rate of CF₄, it should preferably be atleast about 10%, more preferably at least about 16%, and most preferablyat least about 22% of the total gas flow.

The total pressure of this etching process preferably ranges from0.001-0.5 torr, more preferably 0.01-0.3 torr., with the most preferredrange being 0.05-0.25 torr. As for the magnetic gauss level, it can bepreferably be at a set point range of 35-150 gauss.

The multilayer structure of the present invention generally includes asilicon wafer. Preferably, the temperature of the silicon wafer duringthe etching process is important in producing high selectivity ofsilicon dioxide to silicon nitride. It is also important in theformation of a good profile. It has been determined in the subjectprocess that when higher etch temperatures are employed, the highselectivity previously described herein can be readily maintained. Forexample, in the case of certain preferred systems such as the MERIEsystem, a preferable temperature range of the silicon wafer in themultilayer structure during the etching step is about 20-80 degrees C.,more preferably about 30-60 degrees C., and most preferably about 35-50degrees C. This is the temperature of the bottom electrode adjacent tothe silicon wafer location during the etching process.

In the process of this invention the high level of selectivity of theSiO₂ etch rate with respect to said Si₃ N₄ etch rate is preferably atleast about 10:1, more preferably at least about 20:1, and mostpreferably at least about 50:1. The process also produces a preferredhigh SiO₂ etch rate which is at least about 2500 angstroms of SiO₂ perminute, more preferably at least about 3000 angstroms of SiO₂ perminute, and most preferably at least about 4000 angstroms of SiO₂ perminute. Furthermore, the selectivity of the SiO₂ etch rate with respectto the Si₃ N₄ etch rate for etching the silicon dioxide outer layer tothe silicon nitride stop layer, employing a fluorinated chemical etchantsystem including an etchant material and an additive material, ispreferably at least about 500%, more preferably at least about 1000%,and most preferably at least about 1500%, higher than the selectivity ofsaid SiO₂ etch rate with respect to said Si₃ N₄ etch rate for etchingthe silicon dioxide outer layer to the silicon nitride stop layer,employing a fluorinated chemical etchant system including theabove-described etchant material, but without the subject additivematerial.

The process of the present invention preferably includes the step ofetching the exposed areas of the silicon dioxide outer layer down to thesilicon nitride stop layer employing a dry etching process conducted ina magnetically-enhanced etching chamber, more preferably an RIE or anMERIE etching chamber.

The foregoing and other objects, features and advantages of theinvention will become more readily apparent from the following detaileddescription of a preferred embodiment which proceeds with reference tothe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial representation of a multilayer structure of thepresent invention, such as a semiconductor profile, having a silicondioxide outer layer on a silicon nitride etch stop layer, prior toetching with the fluorinated chemical etchant system of the presentinvention.

FIG. 2 is a pictorial representation of the multilayer structure of FIG.1 after etching the silicon dioxide outer layer down to the siliconnitride etch stop layer using the fluorinated chemical etchant system ofthe present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The inventive process herein is directed towards anisotropically etchinga multilayer structure comprising a silicon dioxide outer layer on anunderlying silicon nitride stop layer. Referring now to FIG. 1, aschematic representation of a multilayer structure, which is formed byconventional deposition techniques, is depicted. The multilayerstructure of FIG. 1, generally designated as "10", is shown prior toconducting the subject etching operations. The multilayer structure 10comprises a plurality of structural layers which are sequentiallydeposited onto an underlying silicon structure 18. Multilayer structure10 comprises a plurality of structural layers including an outer layer14 having a major outer surface 14a. Structural layer 14 is fabricatedof SiO₂. Basically, SiO₂ (oxide) can be described as being eitherundoped or doped glass. In the semiconductor industry, the term oxide isgenerally used instead of glass. Generally an undoped oxide is either afield oxide or gate oxide which is usually grown in a furnace. Dopedoxide include BPSG, PSG, etc. which are generally deposited on thesilicon wafer with a dopant gas(es) during a deposition process.

The outer structural layer 14 is deposited onto an adjacent intermediatestructural layer 16. Layer 16 includes sidewalls and is fabricated of anetch stop layer of silicon nitride. Also shown in FIG. 1 is a chemicaletchant protective patterned layer 12 which comprises a photoresistlayer having a predetermined arrangement of openings 12a for forming apredetermined pattern in multilayer structure 10. Typically, this isaccomplished using a semiconductor photomask and known conventional etchmask patterning techniques. The etch stop layer is deposited onto fieldoxide 15, silicon substrate 18, and onto a plurality of polysiliconlines 17 having located adjacent their respective sidewalls spacerelements 19.

As seen in FIG. 2, preferred manner of etching of the SiO₂ structuralSiO₂ layer 14 down to etch stop layer 16 is by plasma etch. The gasplasma etch technique employed herein typically has an etching area in aplasma and is generated under vacuum within the confines of an RFdischarge unit. The preferred plasma etch technique employed herein mayinclude the use of ECR, Electron Cyclotron Resonance, RIE, MIE, MERIE,PE reactive ion, point plasma etching, magnetically confined helicon andhelical resonator, PE, or magnetron PE. In plasma dry etchers, typicallythe upper electrode is powered while the lower electrode is grounded. InRIE (Reactive Ion Etchers), the lower electrode is powered while theupper electrode is grounded. In triode dry etchers, the upper and lowerelectrodes can be powered as well as the sidewall. In MERIE(magnetically enhanced reactive ion etch) magnets are used to increasethe ion density of the plasma. In ECR (Electron Cyclotron Resonance),the plasma is generated upstream from the main reaction chamber. Thisproduces a low ion energy to reduce damage to the wafer.

A semiconductor device can then located in the desired etcher, within anetching area, and is etched with a fluorinated chemical etchant systemto form a predetermined pattern therein. The fluorinated chemicaletchant system comprises a chemical etchant composition of the typedescribed above such as CHF₃ --CF₄ --Ar, and a CH₂ F₂ additive material.The fluorinated chemical etchant system is in a substantially gas phaseduring the etching of the multilayer structure.

The exposed SiO₂ layer is selectively etched at a relatively high etchrate down to the Si₃ N₄ etch stop layer by removing predeterminedportions of the SiO₂ layer by chemically enhanced ionic bombardment.Some areas of the wafer continue to have SiO₂ available to be etchedwhile other areas of the wafer have already reached the nitride layerwhere the etching process effectively stops because of polymer formationon the nitride surface. In this way, the etching process can provide forthe formation of the upright sidewalls in etched layers which have aprofile which is substantially vertical.

EXAMPLE 1

A preferred etching system which is employed in the process of thisinvention is the Applied Materials Precision 5000, a single wafer plasmaetching apparatus manufactured by Applied Materials of Santa Clara,Calif. This apparatus comprises a mobile, double cassette platform, atransport chamber with an 8 wafer storage elevator, and from 1-4 plasmaetching chambers.

The mobile cassette platform is maintained at atmospheric pressureduring the entire operation of the apparatus. It holds two cassettes ofwafers, each capable of holding up to 25 wafers. The platform can beraised or lowered and moved laterally so that any particular wafer maybe lined up with a narrow door between the platform and the transportchamber.

Nitrogen gas is fed through a flow control valve into the transportchamber to vent the chamber to atmosphere. A robot transfer arm in thetransport chamber transfers wafers from the cassette on the mobilecassette platform to the storage elevator in the transport chamber. Thetransport chamber is connected to a two stage evacuation pump which isused to evacuate the transport chamber and a maintain it at a suitablepressure for transporting wafers from the elevator to the plasma etchingchamber. This pressure was maintained at 75-125 mTorr.

The plasma etching chamber is connected to a turbo pump and the twostage pump which evacuates the chamber to a lower pressure than that ofthe transport chamber. This pressure was typically less than 10 mTorr.When the transport chamber and the plasma etching chamber have reachedsuitable pressures for wafer transfer, the robot arm transfers a waferfrom the wafer storage elevator to the plasma etch chamber.

The plasma etching chamber contains an upper, electrically groundedelectrode which also serves as the chamber sidewalls, and a lower, RFpowered electrode upon which the wafer is clamped during the plasma etchprocess, and a set of electromagnetic coils placed around the chambersidewalls. The chamber also contains a gas distribution plate connectedto the lid of the chamber, through which suitable feed gas mixtures arefed into the chamber from a connected gas supply manifold.

When RF energy is applied to the lower electrode, the gas fed into thechamber via the gas distribution plate is converted to plasma. Theplasma contains reactive chemical species which etch selected unmaskedportions of the wafer clamped to the lower electrode. Electric power isapplied to the electromagnetic coils which surround the chambersidewalls. The magnetic field generated by the coils increases thedensity of the plasma near the wafer surface. A throttle valve locatedbetween the plasma etching chamber regulates the pressure of the chamberto processing values, generally in the range of 10-350 mTorr.

The lower electrode is connected to a wafer cooling system designed tomaintain the wafer at a constant temperature during the plasma etchprocess. This system consists of three parts. The first is an apparatusproviding a temperature controlled fluid which circulates through atunnel in the lower electrode. The second part is an apparatus providinga pressure and flow controlled inert gas (typically helium) of highthermal conductivity which is fed to the underside of wafer during etchvia a channel through the lower electrode, opening to grooves on the topface of the lower electrode. The third part of the wafer cooling systemis an o-ring seal which lies partially in a circular groove in the lowerelectrode. The lower electrode is constructed in such a way that it maybe raised so that the wafer placed on its top surface is held against aclamp ring supported above the wafer. When the lower electrode is raisedto clamp the wafer against the clamp ring, the wafer underside is heldtightly against the o-ring seal. This seal prohibits leakage of theinert gas from underneath the wafer to the plasma etch cavity.

The machine is governed by a programmable computer that is programmed toprompt the machine to evacuate and vent the transport chamber and plasmaetching chamber, transfer wafers to and from the cassettes, elevator,and etch chamber, control the delivery of process gas, RF power, andmagnetic field to the plasma etching chamber, and maintain thetemperature of the wafer in the plasma etching chamber, all atappropriate times and in appropriate sequence.

A multilayer structure is then located within the plasma etching chamberand is etched with a fluorinated chemical etchant system to form apredetermined pattern therein. The fluorinated chemical etchant systemcomprises a chemical etchant composition, such as CHF₃, CF₄ and Ar, andan additive material as described above. The fluorinated chemicaletchant system is in a substantially gas phase during the etching of themultilayer structure.

In the case of the chemical etchant composition including CHF₃, CF₄ andAr, and an additive material comprising CH₂ F₂, the exposed SiO₂ layeris selectively etched at a relatively high etch rate and highselectivity down to the Si₃ N₄ etch stop layer by removing predeterminedportions of the SiO₄ layer using chemically enhanced ionic bombardmentof the gas phase etchant material. Some areas of the wafer continue tohave SiO₂ available to be etched while other areas of the wafer havealready reached the nitride layer where the etch process effectivelystops because of polymer formation on the nitride surface. In this way,the etching process can provide for the formation of the uprightsidewalls in etched layers which have a profile which is substantiallyvertical.

Representative etch parameters were employed in the process for etchinga multilayer structure of the present invention, as set forth above inthis EXAMPLE 1. The flow rates of the component gases, based on thetotal gas flow of the fluorinated chemical etchant system, used hereinwas as follows: an etchant material comprised of 16% CF₄, 57% Ar, and 9%CHF₃, at a total pressure in the system of 200 mTorr, magnetic gasesmaintained at 150 gauss, and RF power applied at 500 watts.

When 20% of the total gas flow of CH₂ F₂ was employed as the additivematerial, a silicon dioxide to-silicon nitride selectivity of more than30:1, and a silicon dioxide etch rate of over 4,000 angstroms per minuteresulted.

EXAMPLE 2

When the process of EXAMPLE 1 was repeated as described above, exceptthat no additive material was introduced along with the etchant materialin the feed gas, the selectivity was determined to be about 1.2:1, andthe silicon dioxide etch rate was also about 4000 angstroms per minute.

Therefore, when the process of the present invention was employed inEXAMPLE 1, a selectivity of greater than 30:1 was achieved, as comparedan selectivity of 1.2:1 for the process of EXAMPLE 2. This is anincrease in selectivity of greater than 2400%. In spite of thisoverwhelming disparity in selectivity, the processes of EXAMPLES 1 and 2each had a silicon dioxide etch rate of about 4000 angstroms per minute.

For purposes of the subject invention, including the above EXAMPLE 1 and2, silicon dioxide and silicon nitride wafers were patterned with etchmasks having the appropriate etch mask openings and geometries. Thewafers were then etched at a specific time to effect an etch into therespective films. The wafers were prepared for analysis using a ScanningElectron Microscope (SEM). The amount of film removed was then evaluatedfrom the SEM photomicrographs obtained at the appropriate mask locationsand the etch rates of the respective films were determined. The etchrates were determined by dividing the measured etch depth by the etchtimes. The selectivity was determined by dividing the silicon oxide etchrate by the silicon nitride etch rate.

Having illustrated and described the principles of our invention in apreferred embodiment thereof, it should be readily apparent to thoseskilled in the art that the invention can be modified in arrangement anddetail without departing from such principles. We claim allmodifications coming within the spirit and scope of the accompanyingclaims.

We claim:
 1. A process for plasma etching a multilayer structure to forma predetermined etched pattern therein, comprising:providing amultilayer structure, the outer layers of the multilayer structurecomprising a silicon dioxide outer layer on an underlying siliconnitride stop layer; forming on the top surface of the multilayerstructure a chemical etchant resistant layer having a pattern ofopenings in therein and thereby exposing areas of the silicon dioxideouter layer corresponding to the pattern of openings; and etching theexposed areas of the silicon dioxide outer layer to the silicon nitridestop layer, at a high SiO₂ etch rate, and at a high level of selectivityof said SiO₂ etch rate with respect to said Si₃ N₄ etch rate, with afluorinated chemical etchant system including an etchant material ad anadditive material, said additive material comprising a fluorocarbonmaterial in which the number of hydrogen atoms is equal to or graterthan the number of fluorine atoms, and sad etching step forming an etchpattern in the silicon dioxide outer layer in which the contactsidewalls of said SiO₂ outer layer are perpendicular to the multilayerstructure layers.
 2. The process of claim 1, wherein said additivematerial comprises CH₂ F₂.
 3. The process of claim 1, wherein saidetchant material comprises at least one of CHF₃, CF₄ and Ar.
 4. Theprocess of claim 1, wherein said higher level of selectivity of saidSiO₂ etch rate with respect to said Si₃ N₄ etch rate is at least about10:1.
 5. The process of claim 1, wherein said high SiO₂ etch rate is atleast about 2,500 angstroms of SiO₂ per minute.
 6. The process of claim1, wherein said additive material comprises CH₃ F.
 7. The process ofclaim 1, wherein said etching of the exposed areas of the silicondioxide outer layer to the silicon nitride stop layer comprises a dryetching process conducted in a magnetically-enhanced etching chamber. 8.The process of claim 1, wherein said fluorinated chemical etchant systemcomprises from about 70-90% of said etchant material, and from about10-30% of said additive material, based on the total flow of thefluorinated chemical etchant system.
 9. The process of claim 1, whereinthe selectivity of said SiO₂ etch rate with respect to said Si₃ N₄ etchrate for etching the silicon dioxide outer layer to the silicon nitridestop layer, employing a fluorinated chemical etchant system including anetchant material and an additive material, is at least about 500% higherthan the selectivity of said SiO₂ etch rate with respect to said Si₃ N₄etch rate for etching the silicon dioxide outer layer to the siliconnitride stop layer, employing a fluorinated chemical etchant systemincluding said etchant material without said additive material.
 10. Theprocess of claim 1, wherein said multilayer structure includes a siliconwafer, and the temperature of said silicon wafer during said etchingstep is from about 20-80 degrees C.
 11. A process for using an etchantmaterial for plasma etching a multilayer structure to form a etchedpattern therein, comprising:providing a fluorinated chemical etchantsystem including an etchant material and an additive material, saidadditive material comprising at least one of CH₂ F₂ or CH₃ F; providinga multilayer structure having a plurality of structural layers, one ofthe outer structural layers of the multilayer structure comprising asilicon dioxide outer layer on an underlying silicon nitride stop layer;forming on the top surface of the multilayer structure a chemicaletchant resistant layer having a pattern of openings therein and therebyexposing areas of the silicon dioxide outer layer corresponding to thepattern of openings; and etching the exposed areas of the silicondioxide outer layer to the silicon nitride stop layer, at a high SiO₂etch rate, and at a high level of selectivity of said SiO₂ etch ratewith respect to said Si₃ N₄ etch rate, with a fluorinated chemicaletchant system including an etchant material and an additive material,said additive material comprising a fluorocarbon material in which thenumber of hydrogen atoms is equal to or greater than the number offluorine atoms, ad said etching step forming an etch pattern in thesilicon dioxide outer layer in which the contact sidewalls of sad SiO₂outer layer are perpendicular to the multilayer structure layers. 12.The process of claim 11, wherein said etchant material comprising atleast one of CHF₃, CF₄ and Ar.
 13. The process of claim 11, wherein saidhigh level of selectivity of said SiO₂ etch rate with respect to saidSi₃ N₄ etch rate is at least about 10:1.
 14. The process of claim 11,wherein said high SiO₂ etch rate is at least about 2,500 angstroms ofSiO₂ per minute.
 15. The process of claim 11, wherein said additivematerial comprises CH₃ F.
 16. The process of claim 11, wherein saidetching of the exposed areas of the silicon dioxide outer layer to thesilicon nitride stop layer comprises a dry etching process conducted ina magnetically-enhanced etching chamber.
 17. The process of claim 11,wherein said fluorinated chemical etchant system comprises from bout70-90% of said etchant material, and from about 10-30% of said additivematerial, based on the total flow of the fluorinated chemical etchantsystem.
 18. The process of claim 11, wherein the selectivity of saidSiO₂ etch rate with respect to said Si₃ N₄ etch rate for etching thesilicon dioxide outer layer to the silicon nitride stop layer, employinga fluorinated chemical etchant system including an etchant material andan additive material, is at least about 500% higher than the selectivityof said SiO₂ etch rate with respect to said Si₃ N₄ etch rate for etchingthe silicon dioxide outer layer to the silicon nitride stop layer,employing a fluorinated chemical etchant system including said etchantmaterial without sad additive material.
 19. The process of claim 11,wherein said multilayer structure includes a silicon wafer, and thetemperature of said silicon wafer during said etching step is from about20-80 degrees C.
 20. A fluorinated chemical etchant system for plasmaetching a multilayer structure having a plurality of structural layerscomprising a silicon dioxide outer layer on an underlaying siliconnitride layer, the multilayer structure having a chemical etchantresistant patterned layer including a pattern of openings therein,thereby exposing a plurality of areas of the top surface of themultilayer structure corresponding to the pattern of openings duringetching, which comprises:a fluorinated chemical etchant system includingan etchant material and an additive material, said additive materialcomprising a fluorocarbon material in which the number of hydrogen atomsis equal to or greater than the number of fluorine atoms, for etchingthe plurality of exposed areas of the silicon dioxide outer layer to thesilicon nitride stop layer, at a high SiO₂ etch rate, and at a highlevel of selectivity of said SiO₂ etch rate, respect to said Si₃ N₄ etchrate, to form an etched pattern in the silicon dioxide layer in whichthe contact sidewalls of said SiO₂ outer layer are perpendicular to themultiplayer structure layers.
 21. The system of claim 20, wherein saidadditive material comprises CH₂ F₂.
 22. The system of claim 20, whereinsaid etchant material comprising at least one of CHF₃, CF₄ and Ar. 23.The system of claim 20, wherein said high level of selectivity of saidSiO₂ etch rate with respect to said Si₃ N₄ etch rate is at least about10:1.
 24. The system of claim 20, wherein said high SiO₂ etch rate is atleast about 2,500 angstroms of SiO₂ per minute.
 25. The process of claim20, wherein said fluorinated chemical etchant system comprises fromabout 70-90% of said etchant material, and from about 10-30% of saidadditive material, based on the total flow of the fluorinated chemicaletchant system.
 26. The system of claim 20, wherein the selectivity ofsaid SiO₂ etch rate with respect to said Si₃ N₄ etch rate for etchingthe silicon dioxide outer layer to the silicon nitride stop layer,employing a fluorinated chemical etchant system including an etchantmaterial and an additive material, is at least about 500% higher thanthe selectivity of said SiO₂ etch rate with respect to said Si₃ N₄ etchrate for etching the SiO₂ outer layer to the Si₃ N₄ stop layer,employing a fluorinated chemical etchant system including said etchantmaterial without said additive material.
 27. A dry etching process for amultilayer semiconductor wafer, comprising:a) providing asilicon-dioxide layer; b) providing a silicon-nitride layer, locatedunder the silicon-dioxide layer; and c) using a plasma,having anetchant, and a fluorocarbon additive gas in which the number of hydrogenatoms are greater than the number of fluorine atoms, to etch through thesilicon-dioxide layer down to the top surface of the silicon-nitridelayer.
 28. An etching process as in claim 22, further comprising:etchingwith the plasma when the bottom electrode is within a temperature rangefrom 20 to 80 degrees C.
 29. An etching process as in claim 27, whereinthe silicon-dioxide layer etch rate is at least about 2,500 angstromsper minute.
 30. An etching process as in claim 27, whereinthe plasmaetching is dry etching process conducted in a magnetically-enhancedetching chamber.
 31. An etching process as in claim 27, whereintheplasma etching has a selectivity of SiO₂ etch rate with respect to Si₃N₄ etch rate is at least a 15 to 1 ratio.